Kettle reboiler heat exchangers

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

The function of reboilers has already been discussed. These components are essentially heat exchangers that are used to transfer heat to bring the liquid at the bottom of the column to its boiling point (refer also to discussions in Chapter 1). The principle types employed are jacketted kettles, simple kettle type reboilers, internal reboilers, and thermo-syphon reboilers. Examples of each type are illustrated in Figure 7. 

In addition to the type description code there is also a shorthand that is used for classifying heat exchangers. The first element of the shorthand is the nominal diameter, which is the inside diameter of the shell in inches, rounded off to the nearest integer. For kettle reboilers and chillers ii emember the kettle has a narrow end and a fat end), the nominal diame-tci is the port diameter (the narrow end) followed by the shell diameter, each rounded off to the nearest integer. 

Note that in a kettle reboiler, the bottoms product level control valve does not control the level in the tower it controls the level on the product side of the reboiler only. The liquid level on the boiling or heat-exchanger side of the kettle is controlled by the internal overflow baffle. But what controls the tower-bottom liquid level ... 

Figure 8.4. Example of tubular heat exchangers (see also Fig. 8.14). (a) Double-pipe exchanger, (b) Scraped inner surface of a double-pipe exchanger, (c) Shell-and-tube exchanger with fixed tube sheets, (d) Kettle-type reboiler, (e) Horizontal shell side thermosiphon reboiler, (f) Vertical tube side thermosiphon reboiler, (g) Internal reboiler in a tower, (h) Air cooler with induced draft fan above the tube bank, (i) Air cooler with forced draft fan below the tube bank.

Example 8.12 summarizes the results of such calculations made on the basis of data in Heat Exchanger Design Handbook (1983). Procedures for the design of kettle, thermosiphon and forced circulation reboilers also are outlined by Polley (in Chisholm, 1980, Chap. 3). 

FIG. 11 -36 (Continued) Heat-exchanger-component nomenclature, (c) Outside-packed floating-head exchanger. Type AER (d) U-tube heat exchanger. Type CFU. (e) Kettle-type floating-head reboiler. Type AKT. (Standard of Tubular Exchanger Manufacturers Association, 6th ed., 1978.)... 

After the parameters for estimating equipment sizes and the utility parameters are adjusted, and a new steam utility is defined, the simulation units (blocks, modules, or subroutines) are mapped into Aspen IPE. In this case, there is only one distillation unit, Dl, to be mapped. The default mapping results in (1) a tray tower, (2) a shell-and-tube heat exchanger with a fixed tube sheet for the condenser, (3) a horizontal drum for the reflux accumulator, (4) a centrifugal reflux pump, and (5) a kettle reboiler with U tubes. 

Shell and tube heat exchangers To provide lower inventory and intensify, prefer plate to shell and tube exchangers with the highest surface compactness (H). Shell and tube reboilers To provide lower inventory and intensify, prefer thermosyphon reboilers to kettle reboilers. 

Shell-and-tube heat exchangers can be broken down into (1) pipe-coil (2) double-pipe (3) fixed-head, single-pass (4) fixed-head, multipass (5) floating-head, multipass (U-tube) (6) kettle reboiler (7) thermosyphon reboiler and (8) shell nomenclature. These devices can be mounted vertically or horizontally. 

Another type of heat exchanger is a kettle reboiler. Reboilers are energy-balance devices attached to distillation columns to help control temperature. Reboilers have two basic designs ... 

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